We have generated SIVhyifn, a replication-competent SIVDnef expressing human IFN-g. In this virus, expression of IFN-g is driven by the 5ULTR; two in-frame nef start codons were eliminated without altering the env aminoacid sequence. SIVhyifn is more stable, and expresses higher levels of IFN-g, than SIVsv-ifn. We conducted a comparative pathogenesis study in two groups of rhesus macaques intravenously inoculated with 104TCID50 of SIVDnef of SIVhyifn. Cell-associated virus loads were significantly lower for macaques inoculated with SIVhyifn throughout the experiment. Vaccinated animals, and two naive control animals, werechallenged with 100 MID50 of SIVmac251 at 25 weeks postvaccination. Viral loads for all 12 animals were measured at various times postchallenge. The viral load for SIVhyifn-vaccinated animals was significantly lower than for those vaccinated with SIVDnef (p=0.03). One control animal failed to produce antibodies against SIV and died at 12 weeks postchallenge from severe AIDS-related complications. The second unimmunized control generated an antibody response to SIV, but developed similar complications and was euthanized at 18 weeks postchallenge. Virus isolated from PBMCs and LNCs postchallenge was characterized by PCR. Challenge virus was isolated from both control animals at one WPC. The ratios of virus types isolated after challenge gradually changed, from vaccine virus alone to challenge virus only. However, virus replacement progressed more slowly in the SIVhyifn-vaccinated groups. In contrast to previous studies with recombinant and subunit vaccines, no significant anamnestic response against gp1a60 or gag was observed in vaccinated animals until 6 weeks post-challenge. Anti-gp 160 antibody titers increased in a parallel fashion in both vaccinated groups during the 32 weeks postchallenge period. However, there was a significant decline in anti-gag antibody titers in SIVDnef-vaccinated group compared to those of SIVhyifn after 12 weeks postchallenge.